1. Field of the Invention
The present invention is related to an apparatus for driving a liquid crystal, and, more specifically, to an apparatus for driving a liquid crystal display having a matrix-driven color liquid crystal panel in which the liquid crystals forming the individual pixels of the panel are arranged in the form of a matrix.
2. Related Art
The display devices of personal computers and the like include small, lightweight liquid crystal panel displays (hereinafter, referred to as liquid crystal panels).
On the one hand, among the color liquid crystal panels of this type, the active matrix driven thin film transistor (TFT) liquid crystal panel is currently the most popular. In such a TFT liquid crystal panel, a twisted nematic (TN) liquid crystal is used in which the nematic liquid crystal is twisted by 90.degree..
On the other hand, the simple matrix type has not been used so much with the TN liquid crystal since the contrast decreases as the number of pixels to be displayed increases on a larger screen. However, a super twisted nematic (STN) liquid crystal has recently been developed in which the nematic liquid crystal is twisted by about 260 degrees. A color display has been constructed using an STN liquid panel.
As far as a single cell is concerned, the gray scale level is controlled by the voltage level in the TFT liquid crystal. Gray scale control by using pulse width modulation has been proposed for the STN liquid crystal.
However, since the gray scale level is controlled by the voltage level in the TFT liquid crystal and by pulse width modulation in the STN liquid crystal, as described above, sharing of a drive apparatus by both kinds of display panels has been difficult.
That is, there has been a problem in that, if an attempt is made to drive a TFT liquid crystal panel and an STN liquid crystal panel for display by using one common drive apparatus, the gray scale representation has been limited to a maximum of 8 colors (gray scale levels) with a cell construction using RGB and limited to a maximum of 16 colors (gray scale levels) with a cell construction using RGBI. For a further extended gray scale, the only way to increase the number of gray scale levels has been by frame rate modulation of dithering, so that the advantage inherent in the liquid crystal panels could not be exhibited.
Accordingly, at present, to provide a colorful gray scale representation, which is an advantage of liquid crystals, the drive apparatus for the TFT liquid crystal panel and the drive apparatus for the STN liquid crystal panel have been independently developed, which has resulted in an increase in the cost.